Method of fabricating periodic monolayer semiconductor structures by molecular beam epitaxy

ABSTRACT

Suitably modified molecular beam epitaxy (MBF) techniques are used to synthesize single crystal, periodic monolayer superlattices of semiconductor alloys on single crystal substrates maintained below a critical growth temperature. Described is the fabrication of periodic structures of (GaAs) n  (AlAs) m , where m and n are the number of contiguous monolayers of GaAs and AlAs, respectively, in each period of the structure. As many as 10,000 monolayers were grown in a single structure. Also described is the MBE growth of (Al x  Ga 1-x  As) n  (Ge 2 ) m , quasi-superlattice and non-superlattice structures depending on the particular values of n, m and the growth temperature. Waveguides, heterostructure lasers and X-ray reflectors using some of the structures are also described.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 852,845 filedNov. 18, 1977 now U.S. Pat. No. 4,025,329 which, in turn, is acontinuation-in-part of our copending application Ser. No. 671,565,filed Mar. 29, 1976, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the fabrication of periodic semiconductorstructures by molecular beam epitaxy (MBE).

In recent years considerable attention has been given to the proposal ofEsaki and Tsu (IBM J. Res. Develop., Vol. 14, p. 61, 1970) that newquantum-mechanical effects in electrical properties can be realized in astructure having a one-dimensional periodic potential ("superlattice")formed by a periodic variation of alloy composition or of impuritydensity with a period of the order of a few hundred Angstroms (i.e.,less than the electron mean free path). The experimental work of Ludekeet al typifies the state of the art of this relatively new technology.They report (Appl. Phys. Let, Vol. 24, No. 9, 1974) the use of MBE tofabricate a 161/2 period superlattice in which each period consists of a50 Angstrom layer of GaAs and a 50 Angstrom layer of Ga₀.75 Al₀.25 As on(100) GaAs substrates held at a temperature between 520 and 600 degreesC. during deposition.

Quantum-mechanical effects in such superlattices have been investigatedby Esaki et al in earlier work, tunneling experiments, in which negativeresistance was observed. More recently Dingle and Henry (U.S. Pat. No.3,982,207 issued on Sept. 21, 1976) have proposed a tunable, lowthreshold double heterostructure (DH) laser in which the active regionis a modified superlattice.

What has not been done heretofore, however, is to fabricate asuperlattice in which each half period is made up of as little as asingle monolayer; that is, a layer which has a thickness equal to asingle layer of the compound or element making up the layer. In the GaAs(or AlAs) system, for example, such a monolayer is about 2.8 Angstromsthick and comprises a sheet of Ga (or Al) atoms 1.4 Angstroms thickcontiguous with a sheet of As atoms, also 1.4 Angstroms thick. On theother hand, for Ge, a monolayer is a single sheet of Ge atoms about 1.4Angstroms thick. In fact, the state of the art to date is typified by apaper by A. Y. Cho (Appl. Phy. Let, Vol. 19, p. 467, 1971) whichdiscloses the fabriction of a periodic structure of alternating layersof GaAs and Al_(x) Ga_(1-x) As each 1700 Angstroms thick and numerouspatents by Cho (e.g., U.S. Pat. No. 3,915,765) which state that MBE canbe utilized to grow layers having a wide range of thicknesses; forexample, 3 Angstroms (a single monolayer) to 100,000 Angstroms. Butsingle monolayer growth in the Cho patents deals only with a single,isolated layer grown on a substrate. In view of the non-equilibriumnature of the MBE crystal growth process, it is not at all obvious thatcontiguous monolayers of different materials can be deposited with longrange ordering; i.e., over distances large relative to the layerthickness. Nor is it clear what physical phenomena, such as inter-layerdiffusion or surface-roughening, will inhibit or even prevent suchordering and under what growth conditions, if any, the adverse effectsof such phenomena can be alleviated and ordering achieved.

SUMMARY OF THE INVENTION

We have used suitably modified MBE techniques to fabricate superlatticesmade up of as many as 10,000 alternate monolayers of GaAs and AlAs onGaAs substrates. Transmission electron diffraction (TED) studies ofthese samples revealed a critical growth temperature for growth ofcontiguous monolayers. Below this temperature growth occurred in anearly atomically smooth layer mode, while above this temperature,growth was rough on an atomic scale and obliterated the monolayerstructure. The critical temperature, as well as the perfection of themonolayer structure, were found to also depend on crystal orientation.We believe that this experimental work represents the first realizationof single crystal alternate monolayer synthetic material with goodcrystalline quality and long range ordering.

We have, in addition, used the same MBE system to deposite alternatemonolayers of Al_(x) Ga_(1-x) As and Ge₂ but found thatsuperlattice-like structures were formed only when growth occurred belowa critical temperature and only for specific combinations of the numberof monolayers of each constituent.

For convenience we have adopted the following nomenclature to designatevarious forms of superlattices made up of monolayers of GaAs and AlAs:(GaAs)_(n) (AlAs)_(m) means that each period of the superlatticecomprises n contiguous monolayers of GaAs adjacent to m contiguousmonolayers of AlAs. In general, a monolayer superlattice comprises(A)_(n) (B)_(m), where A and B are different materials or are the samematerials with different doping. In addition, the thicknesses of thesub-lattices (A)_(n) and (B)_(m) are critical if the chemistry andphysics of the superlattice are to be substantially different frommerely forming a heterojunction between different bandgap materials Aand B or forming random (homogeneous) alloys of similar materials. Inparticular, in each sub-lattice of each period of a monolayersuperlattice each molecular plane should either be at an interface withan adjacent sub-lattice or no more than one molecular plate removed fromsuch an interface. This condition translates into the followinglimitation on the parameters m and n: n+m must be no greater than about4 (taking in account that fractions of a monolayer are possible,although not preferred.) This condition also means that the maximumthickness of either sub-lattice should be no greater than about 3t_(o),where t_(o) is the monolayer thickness. Thus, the permissible values ofn and m include (1,1), (1,2), (1,3), (2,2), (2,1) and (3,1).

In this nomenclature the 10,000 monolayer structure alluded to abovewould be designated (GaAs)₁ (AlAs)₁. We have also fabricatedsuperlattices with 1≦n≦9 and 1≦m≦5; for example (GaAs)₂ (AlAs)₂ whichdefines a period comprising two contiguous monolayers of GaAs (5.6Angstroms) and two contiguous monolayers of AlAs (5.6 Angstroms).

Note, however, that the subscripts m and n need not be equal. Nor need mand n be integers although preferably they should be. Thus, we havefabricated (GaAs)_(n) (AlAs)_(m) with n=6.13±0.29 and m=3.37±0.41. Thefact that m and n need not be integers gives rise to the concept of afraction of a monolayer. Of course, since a monolayer is by definition alayer or plane of single atoms, it is as thin as physically possible.So, a fraction of a monolayer refers not to thickness but to area orsurface coverage. Thus, if in depositing a single layer of atoms, theatoms adhere to only eighty percent of the growth surface, then eighttenths of a monolayer has been deposited.

(GaAs)_(n) (AlAs)_(m) monolayer superlattices grown by MBE as describedabove have a number of interesting properties and uses. We have foundthat such structures can be used as waveguide cladding to confine light,and therefore find application in a number of DH junction lasers. Inaddition, because the thicknesses of the layers of the superlattice canbe varied on the scale of atomic dimensions, the structure can be usedas a broadband x-ray reflector.

In addition, monolayer superlattices of (GaAs)_(n) (AlAs)_(m) may bepreferred over the corresponding ternary alloy Al_(x) Ga_(1-x) As. Forexample, undesirable clustering of Ga and Al in ternary alloys may beavoided and strains may be modified and made more regular.

As mentioned previously, we have also used modified MBE to fabricatesuperlattice-like structures of (Al_(x) Ga_(1-x) As)_(n) (Ge₂)_(m)(0≦x≦1) depending on the growth temperature and the particular values ofn and m. For reasons as yet unknown, however, when Al_(x) Ga_(1-x) Asmonolayers are deposited on Ge monolayers, the former tend to exhibitisland growth; that is, rather than cover the Ge layer completely, theAl_(x) Ga_(1-x) As layer tends to nucleate locally so that stacks orislands of Al_(x) Ga_(1-x) As grow. Depending on n and m the Gemonolayers can fill in the gaps between islands and cover the islandscompletely so that a superlattice-like structure results. If not,columnar growth occurs and the resulting structure exhibits pipes orcolumns of Al_(x) Ga_(1-x) As which extend essentially verticallythrough the Ge layers. In either case, however, a new synthetic materialresults; i.e., a material having an absorption edge between that of aAl_(x) Ga_(1-x) As random alloy and that of bulk Ge.

With other growth techniques Ge is not miscible in GaAs (or AlGaAs)beyond ordinary doping levels; (i.e., typically a maximum of about 10¹⁹Ge atoms cm⁻³ can be doped into 10²² cm⁻³ (Avogadro's number) of GaAs,or only one Ge atom for every 1000 of GaAs. This corresponds to 0.1% Ge.In contrast, we have achieved new synthetic materials with as much as50% Ge (e.g., n=m=10) and 90% Ge (n=1, m=10).

(Al_(x) Ga_(1-x) As)_(n) (Ge₂)_(m) may be particularly attractive foruse as the active region of a heterostructure junction laser because itsbandedge (about 1.2 ev or 1 μm) corresponds to an optical transmissionwindow of silica fibers.

BRIEF DESCRIPTION OF THE DRAWING

Our invention together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing (not drawn to scale) in which:

FIG. 1 is schematic of an optical waveguide made up of monolayersuperlattices in accordance with one embodiment of our invention;

FIG. 2 is an expanded view of a portion of the (GaAs)₆ (AlAs)₄superlattice A1 of FIG. 1;

FIG. 3 is an expanded view of a portion of the (GaAs)₉ (AlAs)₁superlattice A2 of FIG. 1;

FIG. 4A is a schematic showing superlattice-like growth of (Al_(x)Ga_(1-x) As)_(n) (Ge₂)_(m), n≲2, m≳5;

FIG. 4B is a schematic showing non-superlattice, columnar growth of(Al_(x) Ga_(1-x) As)_(n) (Ge₂)_(m), n≲2, m≲2;

FIG. 4C is a schematic of a DH laser in which the active region is a(GaAs)₁ (Ge₂)₁ monolayer superlattice in accordance with anotherembodiment of our invention; and

FIG. 5 is a schematic of an x-ray reflector using a monolayersuperlattice in accordance with still another embodiment of ourinvention.

DETAILED DESCRIPTION GENERAL TECHNIQUE

Monolayer superlattices in accordance with our invention were fabricatedby sequential epitaxial molecular beam depositions using the general MBEtechniques described by Cho and Arthur in Progress in Solid StateChemistry, edited by G. Somerjai and J. McCaldin (Pergamon Press, NewYork, 1975) Vol. 10, p. 157.

This type of crystal growth on a low index substrate, where atomic stepsare absent, takes place by two-dimensional nucleation of new atomiclayers. The addition of new material to the crystal occurs either byformation of new nuclei on top of existing ones (island growth) or bylateral growth of the already existing nuclei (layer growth). Anintermediate growth regime referred to as island-layer growth may alsooccur.

The study of the crystal growth process on an atomic scale thus requiressome technique for following the formation and sizes of the new atomiclayers as they are incorporated into the film being deposited. This wasachieved in the following manner. First, a periodic superlatticestructure of two constituents which grow epitaxially (e.g., AlAs andGaAs) was introduced into the film by MBE deposition on a single crystalsubstrate [e.g., (100) or (110) GaAs]. Then, the periodicity in thissuperlattice, its degree of ordering and its composition were probed bytransmission electron microscopy (TEM). These parameters formed a recordof the process by which the crystal grew, and enabled us to distinguishbetween an island growth and a layer growth mode (for islands ≳15-20Angstroms). Electronic properties were probed by photoluminescence andoptical absorption.

The general MBE technique for fabricating either (GaAs)_(n) (AlAs)_(m)or (Al_(x) Ga_(1-x) As)_(n) (Ge₂)_(m) entailed the following standardsteps: (1) obtaining a GaAs substrate from commerical sources; (2)cleaning a major surface of the substrate using standard preparationprocedures; (3) placing the substrate in an evacuable metal chamber; (4)reducing the chamber pressure to less than 10⁻⁸ torr; (5) loadingshuttered effusion cells (ovens) with the requisite source materials forgrowth; (6) heating the substrate to about 600 degrees C. to causedesorption of contaminants from the growth surface and then adjustingthe substrate temperature to that desired for growth; (7) with theshutters closed, heating the ovens until the source materials vaporize;(8) opening selected shutters to effect growth until the desired layerthickness was attained; and (9) maintaining an excess of arsenic at thegrowth surface during deposition. In addition, we modified the MBEapparatus in two respects. First, the standard cylindrical liquidnitrogen cooling shroud surrounding pairs of ovens was replaced with onehaving a honeycombed structure. Each oven was placed in a separateopening in the shroud. We found that this structure greatly reducedcontamination in the system. Second, instead of individual shutters foreach oven we placed one or two rotatable discs in front of the ovens.The discs had annular apertures so that upon rotation selected ones ofthe ovens would be unblocked (e.g., Ga) while others would be blocked(e.g., Al). In this manner, the length of time that the shutter was opencould be made to be about one second by controlling the speed ofrotation of the disc and the size of the disc apertures.

In the fabrication of (GaAs)_(n) (AlAs)_(m) monolayer superlattices,ovens containing As, Al, and Ga were used as beam sources, while (100)and (110) oriented GaAs wafers were used as substrates for the growth.Growth rates were about 1 monolayer/sec with several tenths of a secondof interval between Ga and Al beam periods.

In effect, the fabrication process can be described as forming a newcomposition of matter of A (e.g., GaAs) and B (e.g., AlAs or Ge) bydirecting a periodically pulsed molecular beam at a substrate. During afirst part of each period an A-beam is directed at the substrate for atime effective to grow material A having a thickness of n monolayers andduring a second part of each period directing a B-beam at the substratefor a time effective to grow material B having a thickness of mmonolayers.

Critical Temperature: (GaAs)-(AlAs)

With the system as described above we first investigated layerroughening and inter-layer diffusion by depositing on a single substratea plurality of (GaAs)_(n) (AlAs)_(m) superlattices about 7000 to 9500Angstroms thick separated from one another by GaAs marker layers about1800 Angstroms thick. Each superlattice was grown however at a differentsubstrate temperature. More specifically, we deposited a standard GaAsbuffer layer on a (100) GaAS substrate and then deposited the followingsuperlattices identified only by their (n,m) parameters: (1,1) at 610degrees C.; (2,2) at 611 degrees C.; (4,4) at 611 degrees C.; eight(1,1) superlattices at respectively 602 degrees C., 590 degrees C., 581degrees C., 571 degrees C., 561 degrees C., 551 degrees C., 538 degreesC. and 530 degrees C.; (3,3) at 560 degrees C.; (3,1) at 560 degrees C.;and (1,3) at 560 degrees C. In a similar fashion we fabricatedsuperlattices at deposition temperatures ranging between 510 degrees C.and 630 degrees C. for 0.9≦n≦5 and 0.9≦m≦4.

For example, the effects of the substrate temperature on the growthkinetics of the MBE film were investigated for a monolayer structurewith a (GaAs)₁ (AlAs)₁ composition. The film stoichiometry (i.e., n=m=1)was kept constant to within ±3% for four deposition temperatures: 610C., 602 C., 581 C. and 561 C. Intensity densitometer scans of the TEDpatterns for the four deposition temperatures studied differed from oneanother by the position and intensity of the superlattice satellitespots located in between the (200) and (400) exicted Bragg reflections.For a critical deposition temperature of T_(c) =610 C. no superlatticespot was visible and the film displayed a TED pattern similar to that ofan Al₀.5 Ga₀.5 As random alloy. As the deposition temperature wasdecreased to 602 C., some ordering in the superlattice appeared and twoweak, closely spaced superlattice spots were observed. For T=581 C. thespacing and intensity of the two satellites increased and finally forT=561 C., a single satellite, halfway between the (400) and (200)reflections, was seen. The presence of the satellite halfway between themain (h00) spots (for even h values) is characteristic of the (GaAs)₁(AlAs)₁ superlattice. Unfortunately, multiple scattering effects andsaturation effects introduced in photographically recording the TEDpatterns prevents the use of the intensity ratios between the main spotand satellite spots to measure quantitatively the degree of ordering inthe film deposited at 561 C. However, the dark field images of this filmdid indicate that over 50% of the film contained ordered domains withsizes of about 300-500 Angstroms.

The existence of a critical temperature T_(c) above which thesuperlattice is not formed may have several origins. If island growth isdominant at temperatures T≧T_(c), a high density of small islands willresult on the growth surface which would then exhibit roughening. Thesuperlattice will give a TED pattern similar to a random alloy withcomposition Ga₀.5 Al₀.5 As. This roughening of the growth interfacecould correspond to the original suggestion of Burton and Cabrera thatsingular stepless surfaces undergo a roughening transition at atemperature T which is marked by singularities in the structural andthermodynamic properties of the surface. Recent theoretical Monte Carlosimulations of crystal growth processes have also confirmed theexistence of a transition temperature characteristic of a surfaceroughening during growth. It is also possible that for T>T_(c)interdiffusion between the GaAs and AlAs layers becomes important; theresulting film would again exhibit the TED pattern of a random Ga₀.5Al₀.5 As alloy. However, we note that if interdiffusion at the growthinterface were taking place, it would occur homogeneously throughout thesuperlattice. Thus, a uniform distribution of ordered and disordereddomains would be present in the film. A dark field micrograph of a(GaAs)₄ (AlAs)₄ superlattice deposited at T=610 C. showed thatdisordered domains were preferentially elongated along the growthdirection and were randomly distributed throughout the film as wouldoccur if random rough regions had been nucleated and grown over.Alternate monolayer superlattices grown at T=600 C. exhibit identicalgrowth characteristics. Furthermore, diffusion experiments on MBEmonolayer superlattices have indicated that the bulk diffusioncoefficient, D, for Ga and Al is extremely small [for example, the(GaAs)₁ (AlAs)₁ superlattice is unaffected by an annealing of 2 hours atT=650 C. in an As atmosphere]. Thus, the disordering of the superlatticeis attributed to the surface roughening and the dominance of the islandgrowth mode during the MBE growth when the substrate temperature islarger or equal to T_(c).

The variations in superlattice periodicity shown for films growth atT<T_(c) could possibly be accounted for by: (a) small fluctuations inthe beam stoichiometry (although these were ≦3% for these films duringthe deposition run) and (b) changes in the nucleation and growthkinetics of the islands. The errors involved in measuring both thevariations in the beam stoichiometry and the superlattice periodicity donot permit one to distinguish unambiguously between these twopossibilities. However, our results of similar growth kinetics studieson misoriented substrates [2-3 degrees off (100)] indicated that thesuperlattice periodicity is constant over a large temperature range 550≦T≦590 C. This would suggest that for singular (100) substrates theisland nucleation kinetics are a sensitive function of the depositiontemperature while growth on vicinal planes is controlled by the presenceof a regular array of atomic steps. The TED patterns of the films grownat 570 C. and 560 C. supported the idea that a "layer growth" regime wasdominant during growth.

Intensity densitometer scans of TED patterns were also investigated fora series of superlattice films grown with m>n and m<n and for varioussubstrate temperatures. Superlattice films which were rich in AlAs[e.g., (n,m)=(1,3)] exhibited a critical temperature T_(c) ≃610 C. whilefor superlattices rich in GaAs [e.g., (n,m)=(3,1)], the criticaltemperature was larger than 630 C. The presence of satellite spots in(GaAs)₂.86 (AlAs)₀.91 structures (nominally n3, m=1) grown at 610 C. and630 C. was taken as an indication that a substantial amount of thesuperlattice was ordered. Dark field images of these two structuresformed with superlattice reflections indicated the presence ofdisordered domains. The TED patterns of (GaAs)₁.1 (AlAs)₃.61 and(GaAs)₃.31 (AlAs)₁.22 superlattices deposited at 560 C. exhibited sharp,well-defined satellites and therefore were ordered independent of theratio m/n.

These results underline the strong dependence of T_(c) on the filmchemical composition. It also follows that surface roughening increasesduring the deposition of the AlAs layers. In fact, the surface smoothingeffect during the deposition of the GaAs atomic layers has been used atthe beginning of deposition whenever a structure of stacked alternatingsuperlattices and marker layers was desired. Diffraction data indicatedthat the nucleation and growth of islands control the growth process ofthe AlAs layers deposited at T≳610 C., whereas these island kinetics donot appear to be a controlling factor for the GaAs layers at thistemperature. The difference in the growth process of the GaAs and AlAslayers is not presently understood although the gettering of impuritiesby the highly reactive AlAs surfaces could explain the easy nucleationof islands on these surfaces. Finally, we note that for depositiontemperatures near T_(c), the interface roughness is proportional to theamount of disorder in the superlattice. For example, in a (GaAs)₄(AlAs)₄ superlattice deposited at 610 C., the interface roughness wasoften approximately equal to the dimension of the ordered domains in thedirection orthogonal to the [100] growth axis.

In summary, the growth kinetics of (GaAs)_(n) (AlAs)_(m) superlatticesdeposited on (100) GaAs substrates have shown that:

(a) The substrate temperature strongly influences the growth kinetics ofthe MBE films. The desired layer growth mode appears to dominate overthe temperature range 560 C.≦T≦570 C. for m=n=1.

(b) There exists a critical substrate temperature, T_(c), above which anordered superlattice cannot be formed. For (GaAs)₁ (AlAs)₁superlattices, T_(c) ≃610 C. This transition temperature has beenattributed to roughening of the growth surface.

(c) The film chemical composition strongly affects the value of T_(c) ;furthermore, the value of T_(c) increases in superlattices with n/m>1.Thus, for (n,m)=(3,1), T_(c) ≳630 C. whereas for (n,m)=(1,3), T_(c) ≲610C.

These results point out the importance of the deposition parameters onthe quality of the interfaces in superlattice films deposited by MBE. Itis probable that these interfaces also conrol the electronic propertiesof such structures and thus are of importance for the attainment of MBEsuperlattice type material suitable for device applications.

EXAMPLE: (GaAs)-(AlAs)

We used (100) GaAs substrates held at 577 degrees C to fabricate anumber of other (GaAs)_(n) (AlAs)_(m) monolayer superlattices. Shadowsof the Ga and Al beams were produced by a clip over the substrate andyielded separate measurements of the total deposited GaAs, AlAs andcomposite thicknesses. Glancing incidence 3 keV electron diffractionallowed monitoring of the crystal surface structure during the growth.Under the above growth conditions, an excellent diffraction pattern withlines characteristic of high quality, smooth single crystal (100) layerswith a (2×8) surface reconstruction was observed. Total thicknesses of 1μm to 4 μm of material were deposited. During and after deposition, thesample surfaces remained smooth and featureless. A number of specimenswith different stacking sequences and thicknesses were made, three ofwhich are characterized in Table I.

    ______________________________________                                        Superlattice                                                                  Sample      n          m          n + m                                       ______________________________________                                        A1          6.13±.29                                                                              3.37±.41                                                                              10.0±0.4                                 A2          8.04±.38                                                                              1.15±.14                                                                              10.0±0.4                                 B           1.22±.08                                                                              0.98±.08                                                                              2.25±.06                                 C           1.00±0.1                                                                              1.00±0.1                                                                              2.00±.06                                 ______________________________________                                    

As shown in FIGS. 1-3, sample A comprised a 1.3 μm thick centralsuperlattice A2 of nominally (GaAs)₉ (AlAs)₁ bounded on either side bytwo 1.3 μm thick cladding superlattices A1 of nominally (GaAs)₆ (AlAs)₄.The actual measured values of m and n are given in Table I. Thisstructure could be utilized as an optical waveguide analogous to aconventional DH configuration of random alloys of the same averagecomposition; namely, a Ga₀.9 Al₀.1 As core bounded by Ga₀.6 Al₀.4 Ascladding.

Sample B, on the other hand, comprised 3.6 μm of epitaxial growth with adeposition sequence of 1.22±0.08 monolayers GaAs-0.98±0.08 monolayersAlAs for a total of 6000 periods. Sample C was analogous to sample B butm and n were closer to unity.

As before, TEM was used to investigate the resulting material and testfor the achievement of the monolayer structure. Because of the variouspossible growth modes discussed previously, the programmed superlatticestructure (i.e., that intended by the shutter sequencing) may not beformed in the deposited epitaxial film depending on the growthtemperature and the particular values of (n,m). The actual structuredeposited was determined by transmission electron diffraction (TED) ofthe same area of the film as that used to measure the superlatticeperiodicity λ_(p). For films deposited on (100) GaAs substrates,however, the TED patterns showed satellite spots which werecharacteristic of the atomic order of the superlattice with periodicityλ_(p) =(m+n)d.sub.(200) where d.sub.(200) is the interatomic distancealong the growth direction. The superlattice satellites were most easilyimaged when the film was oriented to excite the (200) or (400) Braggreflections. The sum (m+n) is then given by (m+n)=R.sub.(200) /r_(s)where R.sub.(200) and r_(s) are respectively the distance of the (200)Bragg spots and the satellite spot from the transmitted beam spot.Values of (m+n) measured by this method were obtained with an accuracyof about 1%. Thus, the TED pattern provided a means of verifying whetherthe superlattice period in the film corresponded to that programmed bythe shutter opening and closing sequence.

The monolayer superlattice samples were prepared for TEM examination tooptimize the diffraction effects between the electron beam and thesuperlattice. This condition was achieved when the incident electronbeam is parallel to the multilayer interfaces. The well-known"cross-section" technique was used to realize the necessaryconfiguration. The samples were cleaved along (011) or (011) planes andthinned by mechanical polishing and ion milling with 5 keV Ar⁺ ions to athickness of about 1 μm. The specimens were examined under highresolution imaging conditions in microscopes working both at 100 keV and200 keV accelerating voltages. Both diffraction patterns and latticeimaging modes were utilized to examine the structures.

When the samples were tilted in the TEM so that a (200) or (400) 2-beamdiffraction condition was satisfied, satellite diffraction spotscharacteristic of the new superlattice periodicities were observed oneach side of the transmitted and diffracted beams, in addition to thefull diffraction pattern of fundamental spots of the three-dimensionalcrystal structure. The continuous long range ordering in the growthdirection was also confirmed from (i) the absence of microdomains indark field images formed using one of the strong superlatticereflections and (ii) the absence of any satellite splittingcharacterizing possible partially ordered regions except in sample C.From densitometer intensity scans of the diffraction patterns forsamples A2 and B, well-resolved satellites characteristic ofone-dimensional superlattice structure were observed for both samples,providing direct evidence of achievement of the layered structures. Ninestrong satellites of sample region A2 resulted from the thin layer of1.15±0.14 monolayers of AlAs in each deposition cycle. The satellitepattern for region A1 with intensities peaked near the (000), (200), and(400) reflections was produced by 3.37±0.41 monolayers of AlAs eachcycle period. Two satellites were observed midway between the (200) and(400) spots for sample B. The splitting between these satellites isproportional to the small deviation of the 1.22±0.08 GaAsmonolayers-0.98±0.08 AlAs monolayer deposition sequence from an exact(GaAs)₁ (AlAs)₁ monolayer superlattice, which would have produced asingle satellite midway between the (200) and (400) spots. Values of(m+n) for the three samples are given in Table I and are seen to beconsistent with, but more precise than values determined optically fromthe clip shadows.

It was found for all samples that structure factors calculated from themeasured depositional periodicity and the average deposited layercompositions yielded relative intensities in agreement with observation.Structure factors of hypothetical samples with partially intermixedlayers, as might be produced by interdiffusion between layers or roughgrowth, yielded poorer agreement with the observed patterns, withintensities changing roughly as the square of the long range orderparameter. Based on diffraction and sub-lattice fringes we can concludethat the long range order parameters in the growth direction exceed 0.3relative to a perfect structure, and that we have indeed produced longrange ordered monolayer superlattices. An order parameter of 0.3 meansthat about 65% of the Ga atoms were on Ga sites and, therefore, that 35%of the Ga atoms were on Al sites (i.e., 0.65-0.35=0.3).

Optical Properties

The optical properties of a range of monolayer structures have also beeninvestigated. (GaAs)_(n) (AlAs)_(m) superlattice structures with n≈m≈1exhibited an indirect absorption edge near 2 eV at 2K with anaccompanying emission band which peaked some 15 meV below the absorptionedge. In this composition range the indirect absorption edge appearedalso to represent the minimum band gap of these new materials, and wasnear the random alloy value. The energy of the edge was weakly dependentupon n≈m in the range at least up to n=4; i.e., for (GaAs)₁ (AlAs)₁ upto (GaAs)₄ (AlAs)₄. Measured shifts were consistent with square wellenergy level calculations.

Samples with n≧2>m=1, (i.e., superlattices with more Ga than Al perperiod) showed a well-resolved, intense free-exciton absorption peak andan abrupt, strong interband absorption edge. Specifically, compositionA2 (FIGS. 1 and 3) exhibited an exciton peak at 1.675 eV with a maximumabsorption coefficient, α=1.2×10⁴ cm⁻¹. Evidently the minimum band gapis direct in this superlattice, as it is in random Al_(x) Ga_(1-x) As ofthe same average composition.

Luminescence from the edge of these structures (i.e., emitted in theplane of the layers) was polarized, with small energy differences (˜20meV) and variable intensity differences for the electric vector in theplane (σ) or normal to the plane of the layers (π). Polarization ratiosas large as I.sub.σ /(.sub.π ˜10 have been determined demonstrating thatthe electronic properties at the band edge are anisotropic--a propertythat is impressed upon the structure by the superlattice and that cannotarise in a random Al_(x) Ga_(1-x) As alloy.

Finally, 200 Angstrom GaAs quantum wells with (GaAs)_(n) (AlAs)_(m)monolayer superlattice barriers rather than Al_(x) Ga_(1-x) As barriersshow sharp quantum bound state optical absorption spectra. Thus, themonolayer superlattice is also capable of confining electrons and holes,as is desired in many heterostructure devices. Optical waveguiding inmonolayer structures has also been observed in structures of the typeshown in FIG. 1. Using an incoherent light source from a monochromator,transmission at 1.1 eV to 1.4 eV through superlattice A2 was measured.In one sample we found α≈2 cm⁻¹ and fairly sharp absorption edge withoutmuch band-tailing. In another sample like A2, α ranged from 1 cm⁻¹ at1.1 eV to as high as 4 cm⁻¹ at 1.4 eV.

EXAMPLE: (AlGaAs)-(Ge)

We have also used the same technique to deposit sequences of (GaAs)_(n)(Ge₂)_(m) with (n,m) equal to (1.1) and (1,2) on (100) GaAs substrates.Other substrates are also suitable (e.g., Ge, AlGaAs). Again three ovenswere used but this time Ga, As and Ge were the source materials. Ofcourse, subsequent process steps (e.g., long duration or hightemperature annealing) which might cause significant interdiffusion ofthe GaAs and Ge species should be avoided in order to maintain theintegrity of the deposited structure.

In such (GaAs)_(n) (Ge₂)_(m) structures we found that at growthtemperatures at or above 590° C., a new phase (eutectic of spinodal)appeared in the films which prevented the formation of any superlatticefor n and m ≦2. Although the layers grew epitaxially, misfitdislocations were present to accommodate the large lattice misfit at thegrowth temperature.

On the other hand, for n,m≦2 and at a growth temperature of 400° C.,which is below the 450 degrees C. lower limit taught in the prior artfor MBE growth of GaAs, the films also contained the new phase but, inaddition, evidenced columnar growth, (viz. FIG. 4B) which prevented thegrowth of superlattices. However, these films grew epitaxially andcontained few defects and, therefore, represented new syntheticmaterials having optoelectronic properties intermediate those of GaAsand Ge. Indeed, the amount of Ge incorporated in the compositions farexceeded the miscibility (solubility) limit of Ge in GaAs. For example,optical absorption data showed that the GaAs-Ge structures hadcharacteristics distinct from those of bulk GaAs and bulk Ge. Morespecifically, the optical absorption edge of the structures at 1.2 eVwas between that of GaAs (1.4 eV) and Ge (0.7 eV).

An analogous system, (Al_(x) Ga_(1-x) As)_(n) (Ge₂)_(m) was also studiedfor x=0.5. Results indicated the same phenomena (new phase, columnargrowth) for n,m≦3 at growth temperatures of 500°-575° C. as depicted inFIG. 4B. More specifically, m2 monolayers of Ge₂ were deposited on aGaAs substrate (and buffer layer, not shown), but the subsequent attemptto deposit n2 monolayers of AlGaAs resulted in the growth of islands I1,I2, I3 rather than a uniform layer. The amount of material in theislands, however, was equivalent to that contained in n2 monolayers.Then, m2 monolayers of Ge₂ were deposited but did not completely fill inthe gaps between the AlGaAs islands. As a consequence, the tops of theislands continued to act as nucleation sites for subsequently depositedAlGaAs. Because the islands were continuous extensions of one another,columns of AlGaAs resulted as shown in FIG. 4B.

In contrast, for n=1, m=5 or 10 and a growth temperature of 605° C., aquasi-superlattice as shown in FIG. 4A was fabricated. In this case,islands I1', I2', I3' of AlGaAs again formed when n1=1 monolayer ofAlGaAs was deposited on Ge₂. But, subsequent deposition of m1=5 or 10monolayers of Ge₂ completely covered the AlGaAs islands forming fresh,entirely Ge growth surfaces for subsequent deposition of AlGaAs. Then,another deposition of n1 monolayers of AlGaAs formed another set ofislands I1", I2", and I3". As before, the quantity of material in eachgroup of islands (I1'+I2'+I3') and (I1"+I2"+I3") was equivalent to thatin n1 monolayers of AlGaAs. This quasisuperlattice showed no evidence ofa new phase and no evidence of columnar growth. Epitaxial growth wasgood and relatively defect free.

Similar results were achived with (AlAs)_(n) (Ge₂)_(m) systems.Quasi-superlattices were grown at a temperature of 450° C. for n,m≧10,but the films contained 10⁸ --10⁹ cm⁻² of microtwins. For m=n=1 andgrowth temperatures of 500° C. and 575° C. the films exhibited columnargrowth but no new phase was observed.

Note that the absorption edge of the Al_(x) Ga_(1-x) As-Ge structurescan be made to match very closely the transmission window at about1.0-1.1 μm of silica optical fibers. Consequently in one importantapplication shown in FIG. 4C these structures could form the activeregion 20 of a DH laser used to supply 1 μm radiation to silica fibers.The cladding layers 22 and 24, however, need not be superlattices.Rather they can simply comprise any semiconductor material having alower refractive index than region 20 and which can be readilyfabricated and doped. A suitable cladding is Al_(y) Ga_(1-y) As (0≦y≦1;y>x). Since present experimental evidence suggests that the AlGaAs-Gestructures inherently grow as n-type material, one logical configurationis to make the substrate 26 and cladding 24 n-type and the cladding 22p-type. Other configurations including reversal of conductivity typesare also possible, of course.

X-ray Reflector

The foregoing monolayer superlattices, as well as those fabricated frommaterials yet untried, can be used to form a broad band x-ray reflectorof the type shown in FIG. 5. More specifically, on a substrate 10 thereis grown a plurality of superlattices 12 in which the superlatticeperiodicity varies in a prescribed fashion. The manner in which theperiodicity varies is determined by the necessity to satisfy the Braggcondition λ=2d sin θ for a broadband, divergent x-ray source 14 so thatthe x-rays in a range of wavelengths will be reflected to utilizationmeans 16. Thus, if source 14 emits a range of wavelengths Δλ, then for agiven angle of incidence θ, a range of thicknesses Δd each on the orderof atomic dimensions (i.e., monolayers) is needed to simultaneouslysatisfy the Bragg condition for all frequency components. Each dcorresponds to the periodicity of a superlattice. The divergence of thesource 14, characterized by radiation in a range Δθ of incident angles,is compensated for by curving the monolayers of the superlattice. In oneembodiment this is accomplished by cutting or otherwise forming a holeof diameter D in the bottom of substrate 10. In this configuration thelayers bow downwardly in the central portions (not shown) away fromsource 14. The larger D is the more the monolayers will bend. That themonolayers bend is a direct result of the fact that the substrate (e.g.,GaAs) and the superlattices [e.g., (GaAs)_(n) (AlAs)_(m) ] havedifferent lattice constants so that the monolayers are under strain. Aslong as that strain is not otherwise relieved (e.g., by misfitdislocations), the hole in the substrate will allow the strain to bendthe monolayers.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of our invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular the (GaAs)_(n)(AlAs)_(m) superlattice can be used to form the cladding layers of a DHjunction laser in which the active region is either a random alloy ofAl_(x) Ga_(1-x) As or another GaAs-AlAs superlattice containing agreater n/m ratio. The basic configuration is that of FIG. 1. It ispossible that such cladding layers will have higher thermal conductivitythan random alloys of the same average composition.

Finally, many of the foregoing superlattices, quasi-superlattices andvarious compositions were also deposited on (110) GaAs substrates but ingeneral the films exhibited more roughness than those deposited on (100)GaAs probably because of a greater tendency to grow in an island-growthmode rather than the layer-growth mode.

What is claimed is:
 1. A method of fabricating a composition of matterhaving a periodic structure of alternating sublattices of GaAs and AlAscomprising the steps of:(a) mounting a single crystal substrate in anevacuable chamber, (b) reducing the chamber pressure to a subatmosphericlevel, (c) directing a molecular beam of As molecules at a major surfaceof the substrate, (d) during one time period directing at said surface abeam of Ga atoms so as to grow at least one monolayer of Ga on saidsurface and at least one monolayer of As on said Ga monolayer, therebyforming at least one GaAs monolayer as one sublattice of said structure,(e) during a different time period directing a beam of Al atoms at saidsurface so as to grow at least one monolayer of Al atoms and at leastone monolayer of As atoms on said Al monolayer, thereby forming at leastone monolayer of AlAs as the other sublattice of said structure, (f)controlling the duration of said time periods so that the total numberof monolayers of GaAs and AlAs in both sublattices does not exceed four,(g) repeating steps (d), (e), and (f) so as to form said periodicstructure, and (h) during steps (d), (e), (f), and (g) maintaining thetemperature of said substrate below a critical temperature in the rangeof about 610-630 degrees C.
 2. The method of claim 1 wherein duringsteps (d) and (e) a single monolayer of GaAs is grown in said onesublattice and a single monolayer of AlAs is grown in said othersublattice, and in step (h) the temperature of said substrate ismaintained below a critical temperature of about 610 degrees C.
 3. Amethod of claim 2 wherein said substrate is maintained at a temperatureof about 560-570 degrees C.
 4. A method of fabricating an Al_(x)Ga_(1-x) As-Ge (0≦×≦1) composition of matter comprising the steps of:(a)mounting a single crystal substrate in an evacuable chamber, (b)reducing the chamber pressure to a subatmospheric level, (c) heating thesubstrate to a temperature suitable for epitaxial growth, (d) during onetime period directing a first beam of Ge atoms/molecules at saidsubstrate effective to grow a quantity of Ge equivalent to m monolayers,(e) during a different time period directing second beams of Ga and Almolecules at said substrate effective to grow, on the previouslydeposited Ge, islands of Al_(x) Ga_(1-x) As material in a quantityequivalent to n monolayers, (f) repeating steps (d) and (e) to form aperiodic structure, and (g) avoiding subsequent process steps whichwould cause any significant interdiffusion between the Al_(x) Ga_(1-x)As and Ge.
 5. The method of claim 4 wherein the temperature in step (c)and the duration of the time periods in steps (d) and (e) are mutuallyadapted so that during step (d) the deposited Ge monolayers completelycover said Al_(x) Ga_(1-x) As islands to form a fresh Ge growth surfacefor the growth of Al_(x) Ga_(1-x) As islands during a subsequent step(e), thereby forming a quasi-superlattice of (Al_(x) Ga_(1-x) As)_(n)(Ge₂)_(m).
 6. The method of claim 5 wherein steps (d) and (e) arecontrolled so that x=0.5, n=1, m=10, and in step (c) said substrate isheated to a temperature of no more than about 605 degrees C.
 7. Themethod of claim 5 wherein steps (d) and (e) are controlled so tha x=1,m=n=10 and in step (c) said substrate is heated to a temperature ofabout 450 degrees C.
 8. The method of claim 4 wherein steps (d) and (e)are controlled so that said Ge monolayers do not completely cover saidAl_(x) Ga_(1-x) As islands which then act as nucleation sites forsubsequently deposited Al_(x) Ga_(1-x) As.
 9. The method of claim 8wherein steps (d) and (e) are controlled so that x=0.5 and n=m=3.
 10. Amethod of claim 8 wherein steps (d) and (e) are controlled so that x=0and 1>m, n≦2 and in step (c) the temperature of said substrate ismaintained at about 400 degrees C.
 11. The method of claim 8 whereinsteps (d) and (e) are controlled so that x=1, n=m=1 and in step (c) thetemperature of said substrate is maintained at about 500-575 degrees C.